Human babies accumulate a lot of growth in those nine months between conception and birth to give them and their fleshy, complex brains a fighting chance at survival.
How evolution came to grant humans such a rapid prenatal growth rate has never been clear.
Given how critical brain growth is to early human development and that head size, in turn, influences the size of our jaws, researchers suspect that teeth may hold valuable information about our pregnancies. ancestors.
Teeth begin to form around 6 weeks gestation but do not develop their hardened outer layers until the second trimester. From there, the growing layers can keep records of their life history, from weaning to sexual activity.
“Dental remains are the most abundant parts of the fossil record,” says paleobiologist Leslea Hlusko of Spain’s National Center for Research on Human Evolution (CENIEH), making teeth an ideal candidate for solving such biological mysteries, if a relationship between them and the process in question can be established.
The team’s previous research on monkeys revealed that the slower growth of an unborn animal is linked to a lack of development of the third molar. Hlusko, the paleoanthropologist Tesla Monson of Western Washington University, and his colleagues therefore measured the ratios between the length of the third and first molar in primate species still alive today. to get the relative molar size.
They found that prenatal growth rate, head size, and relative molar size indeed all followed the same pattern for all of these primates. So they used this established model to delve into our evolutionary history, analyzing primate fossils spanning between 6 million and 12,000,000 years ago, spanning 13 hominid species.
Cranial and dental remains indicate that prenatal growth rates have increased over the past 6 million years. Along with the fossilized anatomy of the pelvis and head, these findings support the theory that long, human-like pregnancies evolved over the past hundreds of thousands of millions of years, during the Pleistocene.
While primates switched to walking on two legs during the early Pliocene about 5.333 million years ago, signs of which were beginning to be seen in Australopithecus and Ardipithecus fossils, their prenatal growth rates were even more similar to apes alive today than to ours.
But by the evolution of homo erectus in the Lower Pleistocene, around 2,580,000 years ago, there was a definitive change, which was also reflected in their pelvic anatomy.
“Altered pelvic anatomy, endocranial volume, and predicted prenatal growth rates all provide independent evidence supporting human-like pregnancy and birth evolving in the Pleistocene in the later species of Homo, prior to the emergence of Homo sapiens“, writes the team in its article.
These changes coincide with the expansion of grasslands and herbivore populations, which may have provided the Homo gender with the additional resources needed to fuel increased neonatal size and longer maternal investment.
The advances in tools that also occurred around this time may reflect the increasing brain size of our ancestor, as well as the likely evolution of group hunting, which in turn would have provided even more resources.
“This feedback loop may in turn have enabled the evolution of even larger brains and increased cranial capacity later. Homoleading to H. sapiens“, concludes the team.
This research was published in PNAS.